1. Field of the Invention
The present invention relates to a liquid ejection head and to an image forming apparatus, more particularly to a liquid ejection head and to an image forming apparatus, with which it is possible to arrange ejection ports which eject liquid at high density.
2. Description of the Related Art
There is a known type of image forming apparatus which comprises an ink jet head (a liquid ejection head) in which a large number of nozzles (ejection ports) are arranged, and with which an image is recorded on a recording medium by ejecting ink from the nozzles towards the recording medium, while shifting this ink jet head relatively with respect to the recording medium.
This type of ink jet printer generally supplies ink from an ink tank via ink supply conduits to pressure chambers, and, by supplying electrical signals corresponding to the image data to piezoelectric elements so as to drive them, diaphragms which constitute portions of these pressure chambers are caused to be deformed, so that the volumes of these pressure chambers are reduced, thus causing the ink within these pressure chambers to be ejected from the nozzles as liquid drops.
In recent years it has become desired, even with ink jet printers, to produce images of the high picture quality commonly associated with photographic prints. In this connection, it is contemplated to implement such high picture quality, not only by reducing the size of the liquid ink drops which are ejected from the nozzles by making the diameters of the nozzles smaller, but also by increasing the number of pixels per unit area by arranging the nozzles at high density.
Along with providing ink supply conduits in the diaphragms, it is known to provide on the rear sides of the diaphragms (in other words, on the opposite sides of the diaphragms to their sides on which the plurality of pressure chambers are defined) a common liquid-chamber (a reservoir or the like) which supplies liquid to each of the plurality of pressure chambers (see, for example, Japanese Patent Application Publication Nos. 9-226114, 2000-127379 and 2001-179973).
The problem of mutual interference between the nozzles has recently become prominent, along with increase of the density of the nozzles. In other words, when ejecting ink from the nozzles by causing the pressure chambers to be deformed, combined states between the pressure chambers are sometimes set up, and this phenomenon can exert a negative influence on the ejection of ink. This problem of so-called cross-talk becomes more serious as the distance between the nozzles becomes smaller and the density at which the pressure chambers are provided becomes higher.
In order to address this type of cross-talk problem, the present inventor has previously proposed a print head which comprises a flow control device which has a certain reserve capacity with respect to the flow of liquid ink within its ink flow conduits (see Japanese Patent Application Publication No. 2003-39665 (in particular, FIG. 6)). To specify a more concrete form for this type of flow control device, by way of example, there are shown one such device in which gas-tight spaces or bag-shaped hollow spaces are provided in the interiors of the partition walls which separate between the ink conduits, and one such device in which roughening processing is performed on the end surfaces of the partition walls which separate between the ink conduits, and one in which an aperture which communicates with the external atmosphere (a dummy nozzle) is formed by drilling through a portion which constitutes a wall surface of the common ink conduit (or of an individual ink conduit), and so on.
Furthermore, a device has also been proposed in which, in a plate (a spacer plate) which is interposed between a plate in which a plurality of pressure chambers are provided and a plate in which a common liquid chamber (a manifold chamber) which supplements the ink in each of the plurality of pressure chambers is provided, there is formed a recess groove, which opens to the side of the manifold chamber, and which extends over the plurality of pressure chambers (see Japanese Patent Application Publication No. 2002-234155 (in particular, FIGS. 6 and 7)).
Yet further, a device has also been proposed in which, with an ink jet head in which the nozzles are arranged along a single line, in a side wall of an ink supply tank which supplies ink to a plurality of pressure chambers, there is provided a thinned down portion which absorbs fluctuations of the pressures in the plurality of pressure chambers (see Japanese Patent Application Publication No. 2001-179973 (in particular, FIGS. 1 and 2)).
Even if the pressure chambers are provided on the opposite sides of the diaphragms from their sides on which the common liquid chamber is defined, nevertheless, there is the problem that, along with increase of the density of the nozzle array, increase of the density of the electric wiring such as the drive wiring and so on for supplying drive signals to the piezoelectric elements becomes difficult, because the area for implementing electric wiring becomes tight; so that, finally, increase of the density of the nozzle array reaches a limit.
In concrete terms, although, in the case of an ink jet head in which the nozzles are arranged along a single line (for example, see Japanese Patent Application Publication No. 2001-179973), it is acceptable to arrange the drive wiring just one line outwards from the common liquid chamber, by contrast, in the case of an ink jet head of the full line type in which the nozzles are arranged in a two-dimensional array, i.e., in a lattice configuration or in a staggered configuration, it is necessary to arrange the large number of drive wires by leading them out from each of the large number of piezoelectric elements, which are arranged in a two-dimensional array on the lower side of the common liquid chamber, towards the exterior side of the common liquid chamber; so that, due to such problems in implementation, increase of the density of the drive wiring becomes difficult in whatever way it is approached, and moreover it also becomes difficult to increase the density of the nozzle array.
Next, the question of hampering of increase in the density of the nozzle array due to mutual interference between the nozzles (i.e., of so-called cross-talk) will be considered.
With, for example, the ink jet heads which are described in Japanese Patent Application Publication Nos. 9-226114, 2000-127379 and 2001-179973, the construction is such that it is possible to plan the nozzle length L and the supply conduit height H mutually independently, so that it is possible to avoid deterioration of the ejection response characteristic of the nozzles by making the supply conduit height H large while making the nozzle length L small, and moreover it is thereby possible to enhance the recharging characteristic of the ink into the nozzles.
Now, when the supply conduit height H is made large, on the one hand the beneficial effects are obtained that the viscous resistance proportion in the impedance of the supply conduits is reduced, and that it is possible to reduce pressure variations in the supply conduits (which entail disturbances in the ejection characteristics) due to the random consumption of ink by the various nozzles (the amount of ink consumption varies due to differences in the pattern for ejection), but there is also the problem that the inertia of the ink in the supply conduits is undesirably increased. In other words, when the burden of large flow conduits is assumed, the influence of the inertia of the liquid within these large flow conduits due to external vibration (so-called “sloshing”) inevitably becomes undesirably great.
Furthermore, the countermeasure has also been considered of providing a so-called damper which is endowed with a certain liquid capacity, and of making this capacity C large. As such a damper, for example, there have been suggested: a thin portion formed in a side wall of the common liquid chamber, as described in Japanese Patent Application Publication No. 2001-179973; a flow control device, as described in Japanese Patent Application Publication No. 2003-39665; and a recess groove which is arranged so as to extend over a plurality of the pressure chambers, as described in Japanese Patent Application Publication No. 2002-234155.
The previously described countermeasure of setting the nozzle length L and the supply conduit height H independently and optimizing them, or the countermeasure of implementing a damper, are countermeasures which perform optimization of the values (R, L, C) of so-called passive elements, and, since the most suitable values for these passive elements are different according to the conditions for image formation which vary randomly, such as the picture pattern or the print ratio which is to be outputted and the like, accordingly it is necessary to bear in mind the point that, although it is possible to anticipate the beneficial effect of mitigating, to some degree, the pressures upon which each of the pressure chambers exerts its own influence, it is however not possible to anticipate so great a beneficial effect as actually resetting these pressures upon which each of the pressure chambers exerts its own influence.